Transaminases are enzymes which catalyze the transfer of an amine group from one amino acid to another. In humans, two types of transaminases are distinguishable:                ASAT (ASpartate Amino Transferase), AST or GOT (Glutamate Oxaloacetic Transaminase). This enzyme catalyzes the following reaction (i): L-aspartate+α-ketoglutarate→oxaloacetate+L-glutamate;        ALAT (ALanine Amino Transferase), ALT or GPT (Glutamate Pyruvate Transaminase). This enzyme catalyzes the following reaction (ii): L-alanine+α-ketoglutarate→pyruvate+L-glutamate.        
Transaminases are present in all tissues, mainly in the liver and the cardiac muscle, and in the red blood cells. In the event of hepatic damage (necrosis or hepatitis) or myocardial infarction for example, transaminases are released from the liver or the myocardium, respectively, into the bloodstream, causing an increase in the levels thereof in the blood. The assay of transaminases in the serum or the plasma of patients constitutes a tool for the diagnosis of hepatic and cardiac pathologies.
The principal methods for assaying plasma transaminases are based on optical, electrochemical, chromatographic or radiochemical methods (for a review see Huang et al., Sensors, 2006, 6:756-782). Optical methods are routinely used; among these methods, there may be mentioned:
Colorimetry:
This assay method is carried out using serum or plasma. After stopping the enzymatic reactions catalyzed by AST or ALT, a diazonium salt is used which reacts not only with oxaloacetate (one of the products of the reaction catalyzed by AST) to give a compound that is red in color, but also with L-glutamate (one of the products of the reactions catalyzed by AST and ALT) to give, in the presence of glutamate dehydrogenase, a compound that is green in color; the optical density (OD) of the red- or green-colored plasma or serum is then simply measured. The Beer-Lambert law then makes it possible to link the optical density of a solution to its molar concentration (c): OD=εlc, “ε” being the specific coefficient of absorption of the colored compound and “l” the thickness of the cuvette containing the solution (serum or plasma). The disadvantages of this method are mainly the calibration of the cuvettes which has to be carried out for each assay and requires stopping the enzymatic reactions in order to carry out the assay. It is also possible to use this method after immobilizing the reagents on a solid phase, for example on a membrane. Various devices for assaying transaminases, based on a colorimetric method after immobilization of the reagents on a solid phase, have been proposed. Among these devices, there may be mentioned the point-of-care biological device called Cholestech LDX® (Cholestech). The assay of AST and ALT transaminases using this device is carried out on a sample of 35 μL of capillary whole blood, and comprises the following steps: the blood is deposited in the well of a (disposable) cartridge (of the device) and migrates by capillarity in the cartridge which comprises a filter specifically retaining the red blood cells; the resulting plasma is then brought into contact with dry reagents on a membrane, leading to a colorimetric reaction; the optical system of the device then measures the intensity of the final color of the reaction by reflectance photometry, the blue coloration measured having an intensity proportional to the concentration of transaminases present in the sample (international applications WO 2004/90555 and WO 2005/044429);
UV Absorption Spectrophotometry at 340 nm:
According to this method, it is the activity of enzymes which degrade oxaloacetate and pyruvate, produced by AST and ALT respectively, which is measured during secondary enzymatic reactions. These enzymatic reactions are respectively catalyzed by the enzymes MDH (Malate DeHydrogenase; EC 1.1.1.37) and LDH (Lactate DeHydrogenase; EC 1.1.1.27), in the presence of NADH (β-nicotinamide adenine dinucleotide):                MDH catalyzes the following reaction (iii): oxaloacetate+NADH+H+→malate+NAD+;        LDH catalyzes the following reaction (iv): pyruvate+NADH+H+→L-lactate+NAD+.        
NADH (reduced form) absorbs in the ultraviolet (UV) region at 340 nm, whereas NAD+ (oxidized form) does not absorb at this wavelength. It is therefore possible to measure the enzymatic activity of the side reactions catalyzed by LDH or MDH by monitoring the decrease in UV absorbance at 340 nm (corresponding to the consumption of NADH during the enzymatic side reactions as a function of time), and thus deduce therefrom the enzymatic activities of AST and ALT. The assay of the transaminases AST and ALT by this method is carried out, not directly on whole blood, but on serum or plasma (Karmen et al., J. Clin. Invest., 1955, 34:131-133; U.S. Pat. No. 3,925,162 and European patent application EP 415 188). Indeed, whole blood, which comprises red blood cells, is a very absorbent medium, in particular at the wavelength of 340 nm, and variable levels of hematocrit (defined as the ratio, expressed as a percentage, between the volume of red blood cells obtained by centrifuging a sample of whole blood and the volume of this sample) of different patients exhibiting moreover an identical transaminase level could give rise to different transaminase assay results because the variation in UV absorption is thought to be a function of the quantity of red blood cells and not a function of the level of transaminases. In accordance with the recommendations of the IFCC (International Federation of Clinical Chemistry and Laboratory Medicine), the assay of AST according to this method comprises the following steps: mixing the plasma or the serum with L-aspartate, NADH, pyridoxal phosphate (PPal or PLP for pyridoxal-phosphate), an MDH and an LDH in an appropriate buffer; the mixture is incubated for 5 minutes; α-ketoglutarate is then added (so as to initiate the enzymatic reaction (i) above; the mixture is allowed to incubate for 90 seconds; and the decrease in UV absorbance of NADH is measured for 180 seconds (Schumann et al., Clin. Chem. Lab. Med., 2002, 40:725-733). A latency (or incubation) phase of 90 seconds is recommended in order to obtain a linear decrease in the signal (UV absorbance) measured as a function of time. In accordance with the IFCC recommendations, the assay of ALT according to this method comprises the same steps as those described for the AST assay, but no MDH is used and L-aspartate is replaced with L-alanine; the addition of α-ketoglutarate makes it possible to initiate the enzymatic reaction (ii) above (Schumann et al., Clin. Chem. Lab. Med., 2002, 40:718-724).
Fluorescence:
This method involves the same enzymatic side reactions as those described for UV absorption spectrophotometry above. NADH, excited at a wavelength of 340 nm, fluoresces at a wavelength of 460 nm, which is not the case for its oxidized form, NAD+. The intensity of emission is then dependent on the concentration of NADH in the medium studied. The enzymatic activity of the transaminases AST and ALT can therefore be monitored by the reduction in the fluorescence emitted by NADH at 460 nm, either in a homogeneous phase, or after immobilization on a solid support. For the same reasons set out above, the assay of the transaminases AST and ALT by this method can only be carried out on serum or plasma (U.S. Pat. No. 3,925,162 and U.S. Pat. No. 5,612,178; international application WO 91/013169).
The assay of the plasma enzymes which have NADH as cofactor (NADH-dependent enzymes), such as LDHs and MDHs, is carried out, on serum or plasma, by measuring their enzymatic activity by NADH UV absorption spectrophotometry at 340 nm or by monitoring the reduction in the fluorescence at 460 nm of the NADH consumed during the enzymatic reaction catalyzed by these enzymes (in the case of the assay of MDH, it is the NADH consumed during the enzymatic reaction (iii) described above which is measured and in the case of the assay of LDH, it is the NADH consumed during the enzymatic reaction (iv) described above which is measured). The assay of these enzymes is performed by mixing the serum or plasma sample, their substrates (including NADH).
The assaying of LDHs, for example, is used for the diagnosis and monitoring of hepatic and cardiac conditions, and of certain cancers (lung and kidney cancers).